I. Field of the Invention
The present invention relates to silk screen printing methods for imprinting labels carried on a web and the method of transferring the imprinted label to an article. The invention particularly relates to the silk screen printing of heat transferable labels carried on a web and to ink formulations for this purpose.
II. Description of the Prior Art
Heat transferable lables are typically composed of a paper backing overlaid with a release layer typically of wax in turn overprinted with an ink design. A heat transfer label of this type is disclosed in U.S. Pat. No. 3,616,015. Such labels are used to transfer an ink design image from a paper carrier web to the surface of an article, such as a container or bottle. In the transfer process the free side of the paper backing is first heated enough to melt the wax layer coating and tackify the ink design layer, and the label is then rolled or pressed onto the surface of the article so that the ink design makes contact with the article. The paper backing is then released from the molten wax release layer thus allowing transfer of the ink design to the surface of the article. The wax layer forms a protective coating over the ink design after the wax has dried. Heat transfer labels of this type may include other layers such as barrier layers between the ink layer and the release layer to improve the chemical resistance of the design image and/or to improve the release characteristics of the labels. The ink layer may also be overcoated with additional protective layers. Heat transfer labels of this type have received increasing use in industry for imprinting designs onto three-dimensional objects having varying degrees of smooth curvature and, in particular, for imprinting cylindrical articles. Additionally, these heat transfer type labels may be used to imprint designs on flat surfaces for a wide range of materials, including plastic, paper and glass.
Heat transfer labels of the type disclosed in U.S. Pat. No. 3,616,015 have heretofore been manufactured primarily by the use of rotogravure methods for imprinting the various layers including the wax release layer and ink layer onto the paper backing. In particular, it has been customary to coat the ink design layer by rotogravure technique even though the release wax layer may be coated by other methods, such as reverse roll coating techniques. The use of rotogravure for this purpose has been found to have serious economic disadvantages in the production of smaller quantities of heat transferable labels. The capital investment necessary for purchase and etching of a rotogravure cylinder far exceeds that required for purchase and preparation of an imprinted screen for use in the screen printing technique. Although it is more costly and time consuming to prepare a gravure cylinder than an imprinted silk screen, the gravure method is much faster than the silk screen method, once the gravure cylinder has been etched and is on line. Thus the gravure method has been found to be more advantageous for imprinting an ink design onto the release layer when large quantities of the heat transferable labels are to be manufactured using the same design, while there is a distinct economic advantage when employing the silk screen method for production of heat transferable labels in smaller quantities, e.g. less than 250,000 for small sized labels and under 5,000 for very large labels.
Despite the above-described advantages of silk screen printing methods for imprinting an ink design layer onto a release coated paper, such as a wax coated paper, it has heretofore been difficult to produce an ink formulation which is suitable for use in this particular application. The prior art discloses different types of ink formulations, typically conventional oil based or nitrocellulose based inks, as suitable in the silk screen process. The application of such conventional inks onto a wax coated paper backing to form a heat transferable label of the type disclosed in U.S. Pat. No. 3,616,015 presents the difficulties of satisfying an array of physical characteristics which are unique to this application. The silk screen type ink formulations disclosed in the prior art are typically made for application directly from the screen to the surface of either a flat substrate such as a woven or nonwoven material or directly to the surface of a three-dimensional article having smooth curves such as cylindrical objects. U.S. Pat. No. 3,109,365, for example, shows apparatus wherein designs are imprinted directly onto a cylindrical article by the use of a silk screen printing method. There are a number of inks which have been found suitable for the use in this type of conventional silk screen methods wherein the ink is transferred directly through the silk screen onto either a flat or three-dimensional object. Such inks typically may be either an oil-based type ink or a nitrocellulose base ink. Typical formulations are disclosed, for example, in E. A. Apps, Ink Technology for Printers and Students, Vol. 3 (1969), Chemical Publishing Co., Inc., pp. 37-48. Silk screen type inks which are applied directly to the article must have the properties such that the ink does not dry on the screen and does not physically break down upon agitation caused by motion of the squeegee.
When the ink is to be transferred to a release coating such as a wax release there are particular problems to be overcome such as providing the ink with sufficient adhesive bonding characteristics to the wax and yet allowing the ink to be sufficiently pliable and non-brittle after the ink has dried on the release surface. In addition, the ink must have suitable flow characteristics to be squeegeed through the silk screen onto the wax coated paper which forms the release substrate for the heat transferable label. The ink must have exceptional contact characteristics as well as adhesion characteristics so that the ink will lay down smoothly and adhere to the wax surface yet remain flexible and non-brittle upon evaporation of solvents contained in the ink. In addition, the ink must have exceptional adhesive and plastic properties to permit transfer of the ink from the web to an article upon application of heat and pressure. The ink layers for this application should be relatively thick as compared with gravure printing ink layers. When imprinting an ink design onto heat transferable labels it is in fact sometimes desirable to have thicker ink designs than those which are conventionally used when the ink is transferred directly from silk screen to an article. Thus, the present ink formulation must also permit imprinting of thicker designs.
Accordingly, it is an object of the present invention to provide a continuous silk screen process for use in the production of heat transferable labels. Of particular interest is the printing of an ink design onto a release substrate such as a wax coated paper web.
It is another object of the present invention to provide ink formulations which simultaneously exhibit the above-listed array of physical characteristics which make the ink particularly suitable for use in the screen printing of heat transferable labels.